## Functional shock test per mil-std-810 - equivalent static load

## Functional shock test per mil-std-810 - equivalent static load

(OP)

Does anyone have a rule of thumb or calculation for the static equivalent load due to a shock test? This isn't a drop test but a shock simulating abuse during the products life. For example 40g's at 6 ms.

I have always used the g load times the mass of the part and never had any problem but this methodolgy is being questioned and I have no text to reference.

thanks

I have always used the g load times the mass of the part and never had any problem but this methodolgy is being questioned and I have no text to reference.

thanks

## RE: Functional shock test per mil-std-810 - equivalent static load

TTFN

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## RE: Functional shock test per mil-std-810 - equivalent static load

Figure 516.5-10 of the specifcation gives details on a Ideal sawtooth pulse and includes a note which describes the formula: Vi = 0.5*T*P

where: Vi = velocity change

T = time

P = peak acceleration magnitude

What one finds by using this calculation is that the acceleration is basically cut in half.

So heres the rub: When I make some assumptions about crumple zones and velcity/acceleration a 75g shock seems consistent with a vehicle crashing at about 50 MPH. Using half that value seems a little on the low side. In addition I've always used g loading in calcualtions without ever having it brought into question and the analysis was generally reviewed by old timers a lot smarter than me.

Help!?

## RE: Functional shock test per mil-std-810 - equivalent static load

http:/

But, why not use what's in Table 516.5-I?

TTFN

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## RE: Functional shock test per mil-std-810 - equivalent static load

if you assume constant acceleration (deceleration) then the acceleration experienced in stopping from velocity V

_{0}over a distance X isV

_{0}^{2}/ (2 X)in a car crash from 88 ft/sec the front end stops say in 1/2 ft (6 inches) so it senses a deceleration of...

88*88/2/.5 / 32 = 242 g's

the car crushes and the back end stops in 5 feet, so it senses...

88*88/2/5/32 = 24.2 g's.

this of course is a completely inelastic impact.... ignoring elastic shock responses through the car during the event, but you asked for the overall static equivalent shock and the answer seems to be ... well it depends on where you are in the car...

regards

magicme

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there's no place like gnome.

## RE: Functional shock test per mil-std-810 - equivalent static load

so the front end sees 242g's for .0114seconds and the back end sees 24.2 g's for .114 seconds (using an average 44 fps velocity during the crash to calculate time)

magicme

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there's no place like gnome.

## RE: Functional shock test per mil-std-810 - equivalent static load

The MIL-STD gives a formula for the shock accleration but a couple of paticulars that concern me when trying to calculate stress are:

1. I think there are some issues relative to the rate of applied load (ie) strain RATE, with faster being worse.

2. I would think some safety margin would be used to account for the oops factor.

thanks

## RE: Functional shock test per mil-std-810 - equivalent static load

d = A_eq * G / w_n^2

Were:

d = calculated equivalent static deflection in inches

A_eq = equivalent static acceleration in G’s

W_n = natural frequency of system in rad/sec

This equation applies strictly only to a single degree of freedom system, but is a good approximation for a dominant fundamental mode of vibration.

The equivalent static acceleration can be calculated using equations in Chapter 23 of the "Shock and Vibration Handbook", Third Edition. See Equation (23.46) for half sine shock pulse.

These equations strictly apply only in the linear region of response. If the material response is in the plastic region, then you have to account for the energy absorbed by the plastic deformation of the material.

Regards,

G Rudd

## RE: Functional shock test per mil-std-810 - equivalent static load

• Acceleration loads are expressed in terms of LOAD FACTORS

o For acceleration: inertia load factors applied slowly enough and held steady for a period of time long enough such that the material has sufficient time to

• fully distribute the resulting inertia loads, and

• such that dynamic (resonant) response of the material is not excited.

• Shock is

o a rapid motion that excites dynamic (resonant) response of the material, but

o with very little overall deflection (stress)

• Acceleration vs Shock

o Acceleration loads expressed in terms of load factors (g loads)

o Shock environments also in terms of g loads, but

? Acceleration (static) requirements cannot be satisfied by shock (dynamic)

? Shock cannot be satisfied by acceleration

? Acceleration test criteria and methods cannot be substituted for shock test criteria and methods.

Shock test criteria and test methods cannot be substituted for acceleration criteria and test methods

## RE: Functional shock test per mil-std-810 - equivalent static load

• Acceleration loads are expressed in terms of LOAD FACTORS

o For acceleration: inertia load factors applied slowly enough and held steady for a period of time long enough such that the material has sufficient time to

• fully distribute the resulting inertia loads, and

• such that dynamic (resonant) response of the material is not excited.

• Shock is

o a rapid motion that excites dynamic (resonant) response of the material, but

o with very little overall deflection (stress)

• Acceleration vs Shock

o Acceleration loads expressed in terms of load factors (g loads)

o Shock environments also in terms of g loads, but

? Acceleration (static) requirements cannot be satisfied by shock (dynamic)

? Shock cannot be satisfied by acceleration

? Acceleration test criteria and methods cannot be substituted for shock test criteria and methods.

Shock test criteria and test methods cannot be substituted for acceleration criteria and test methods

## RE: Functional shock test per mil-std-810 - equivalent static load

Tunalover

## RE: Functional shock test per mil-std-810 - equivalent static load

I think you are referring to NRL-1396 "Interim Design Values for Shock Design of Shipboard Equipment", May 1980.

that memo gives a series of equations and table for calculating shock "design loads" for equipment, based on it's fundamental frequency, mass, location on the ship, type of ship.

way back, I did plot up some graphs from the equations for quick use... the only one I can find, I posted here...

www.tikmark.com/NRL1396.jpg

at one time I also wrote a neat little calculator for doing these, but can't find it at the moment (it was several computers ago !! )

possibly this is useful

regards

------------------------------------

there's no place like gnome.

## RE: Functional shock test per mil-std-810 - equivalent static load

I couldn't make sense of your attachment. Maybe part of it was cropped off. It's important to note that the G-level depends on the direction of application. Athwartship is different from vertical is different from...Most certainly the vertical values will be greater than all other directions.

Tunalover

## RE: Functional shock test per mil-std-810 - equivalent static load

Another reference to look at is NAVSEA-0901-LP-3010 (I think that's the full designation).

But all of these ship-related documents are based on some assumptions such as that this is a ship floating in water receiving a shock from a mine detonation on a steel ship. The curves depend on whether this is a surface ship or submarine, equipment is hull mounted or deck mounted, which direction, etc.

I didn't get the impression that this situation involved ANY of this, and the development of these equivalent shock loads is inappropriate for anything other than ship-related shock.

Garland E. Borowski, PE

Borowski Engineering & Analytical Services, Inc.

Lower Alabama SolidWorks Users Group

MagnitudeThe Finite Element Analysis Magazine for the Engineering Community## RE: Functional shock test per mil-std-810 - equivalent static load

TTFN

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## RE: Functional shock test per mil-std-810 - equivalent static load

GBor... i think this is the declassified document you refer to ... the original was dated 1963 and was revised in 1980 (maybe also later, but i don't know). and you are correct .... this data applies strictly to shipboard equipment only.

and possibly all of this goes way beyond what the OP asked about ...?

regards,

magicme

------------------------------------

there's no place like gnome.

## RE: Functional shock test per mil-std-810 - equivalent static load

I have always used the g load times the mass of the part and never had any problem but this methodolgy is being questioned and I have no text to reference.”

Hmmm, the people who are asking must not know F=MA. A is Gs (gravity) so bigger Gs times M is bigger force (Lbs).

Multiplying the Gs X Mass is correct to get the static load; however the time translates to the frequency that the 40g will be at full response. The frequency would be 1/(2*.006) = 83 hz. Anything that has a Fn of 83 hz will see 40g plus amplification. With no damping (or 2% damping) and +/- 3 octaves from the Fn, your best case would be that a part will only see 40g. It’s the part that has the same Fn as the shock input is the one you should be worried about.

Tobalcane

"If you avoid failure, you also avoid success."

## RE: Functional shock test per mil-std-810 - equivalent static load

Tobalcane

"If you avoid failure, you also avoid success."

## RE: Functional shock test per mil-std-810 - equivalent static load

Now for the tough part, how do you determine the amplification factor. Well if you can approximate your system as a 1DOF, or 2DOF system, the response of these types of systems can be found in any text on vibrations. In fact you can find them online. There will be plots of amplification factor vs frequency ratio. If you know the frequency ratio you can determine the amplification factor. For instance, assume your structure has a fundamental frequency of 250 Hz and lets say the pule duration is .0085. This translates into 60Hz. Yor frequency ratio is 1.667. From the response plot, the amplification factor is going to be (assuming 5% structural damping) ~1.55. So if your shock input is 40 G's, you would actually multiply 62 G's by the mass of the structure.

However if you have closely spaced modes then there will be dynamic coupling and the amplification can be much, much higer.

If you have access to FEA software you can easily model these things. Bottom line: Unless the natural frequency is on the order of 10X higher than the frequency of the pulse, you need to have some amplification factor.

## RE: Functional shock test per mil-std-810 - equivalent static load

I stand corrected.

NRL7396 (that you refer to) is *not* the NRL1396 that I referred to.

They are separate documents.

regards,

magicme

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there's no place like gnome.

## RE: Functional shock test per mil-std-810 - equivalent static load